I'm a PC in a D&D 5e campaign being run by a friend. During an encounter with some mephits, I used Gust of Wind to try and keep them from swamping the group as we have no one in a tank role. The DM claimed that the pushback effect is only active on the first turn of the spell's activation. As I read the spell description, I would think if that were true there would be a qualifier in the text, but I don't see one.
Gust of Wind
Casting Time: 1 action
Range: Self (60-foot line)
Components: V, S, M (a legume seed)
Duration: Concentration, up to 1 minute
A line of strong wind 60 feet long and 10 feet wide blasts from you in a direction you choose for the spell’s duration. Each creature that starts its turn in the line must succeed on a Strength saving throw or be pushed 15 feet away from you in a direction following the line.
Any creature in the line must spend 2 feet of movement for every 1 foot it moves when moving closer to you.
The gust disperses gas or vapor, and it extinguishes candles, torches, and similar unprotected flames in the area. It causes protected flames, such as those of lanterns, to dance wildly and has a 50 percent chance to extinguish them.
As a bonus action on each of your turns before the spell ends, you can change the direction in which the line blasts from you.
I don't see a qualifier that constrains the pushback effect to just the first turn, and I feel the spell's usefulness is severely affected by this condition. Especially for a level 2 spell slot.
Am I wrong about the spells effect? If I'm not, how best could I convince the DM not to nerf my spell? I'm a Circle of Land druid and feeling very weak in a direct damage sort of way. I was close to feeling useful to the group when I pulled this trick out and I feel like I was undermined arbitrarily.